The hydraulic system for a tipper substantially comprises a pump of constant displacement volume which is driven by the truck engine thus delivering a flow of a speed responsive fluid volume, further a proportional directional control valve of the bypass type which is hydraulically or pneumatically controlled, a load holding valve for the telescopic cylinder which load holding valve is integral with the solenoid valve, a load-sided pressure unloading valve and a pump-sided pressure limiting valve.
The speed of a multiple stage telescopic cylinder for operating the dumper is controlled by the proportional adjustment of the directional valve while the truck engine is idling. The higher the load the smaller becomes the range of fine control as the prior system is load-dependent. When the load pressure reaches the level to which the pressure unloading valve is adjusted the volume displaced by the pump is returned to the reservoir via the pressure unloading valve which is located in the pump pressure line.
The speed of the telescopic cylinder may further be controlled by adjusting the pump displacement volume i.e. by adjusting the pump speed when the directional valve is fully open. However, there is a disadvantage because fluid volumes only can be controlled which are greater than the displacement volume of the pump while the engine is idling, in other words a fine control may be obtained between the minimum and the maximum displacement volume with the maximum speed only.